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Effects of Aphid Densities on Gene Expression in Arabidopsis thaliana
Overview To protect themselves from herbivorous insects, plants must develop a method of pest control and resistance. To accomplish this, many plants produce hormones such as salicyclic acid and jasmonic acid. These hormones produce pathogen-related proteins which allow plants to build a systemic acquired resistance due to their production in the event of an attacking pathogen; an infection in one part of the plant then induces resistance in the rest of the plant, thus creating a "whole-plant" resistance. This resistance may be transferred into surrounding plants if the salicyclic acid is converted into methyl salicylate (an ester). Additionally, in the event of attacking insects, jasmonic acid is released, thus activating the expression of protease inhibitors. The WRKY genes, MYC2, and VSP2 genes all play crucial roles in this defense signaling mechanism.'' WRKY'' genes are important regulators of SA signal-transduction pathways, MYC2 (Jasmonate Incentive 1) is an important transcriptional activator of JA-responsive genes, and VSP2 (Vegetative Storage Protein 2) is a marker gene. In addition to functioning separately, JA and SA signaling pathways for these defense mechanisms are known to interact, or "crosstalk", meaning that various factors work in synchronization to create the plant's defense response. Arabadopsis thaliana A small flowering plant native to Eurasia that serves as a model organism in various experiments thanks to its relatively small genome (~135 megabase pairs). This was the first plant to have its genome sequenced. Pests Brevicoryne brassicae ''(Cabbage aphid) These small insects are destructive aphids that are native to Europe. They normally feed on various types of produce, but will not venture out of the ''brassicae family of plants in a natural setting. Plutella xylostella (Cabbage moth) These moths typically feed only on plants that produce glucosinolates (natural chemicals that contain sulfur and nitrogen; they are water-soluble anions that most likely aid in the plant's defense against pests and diseases) possess a short life cycle (14 days at 25°C). Methods To study the interference of different densities of feeding Brevicoryne brassicae aphids with the plant's induced defensive response against Plutella xylostella caterpillars, scientists utilized two mutant versions of the plant: sid-2, a mutant that cannot induce a SA defensive response; and dde-2, a mutant that cannot induce a JA defensive response. Insect growth was tested on both plants to see if a specific defensive response pathway would affect the population growth of the caterpillars. "To investigate whether aphid feeding interfered with caterpillar-induced defense responses through crosstalk between JA and SA signaling pathways, plants of the sid-2 mutant (a SA induction-deficient mutant) and of the dde-2 mutant (a JA-deficient mutant) were infested with only caterpillars or a combination of caterpillars and a low or high aphid density. Growth of P. xylostella caterpillars was not affected on sid2 or on dde2-2 mutant plants: no significant differences in weight gain of caterpillars feeding during 5 d without or with aphids at a low density (F = 0.205, P = 0.653; Fig. 1B and F = 0.816, P = 0.374; Fig. 1C) or at a high density (F = 0.008, P = 0.931; Fig. 2B and F = 0.820, P = 0.373; Fig. 2C) were found. This indicates that intact SA and JA signaling are both required for the interference by aphids." Results and Conclusions Different densities of aphids affect the expression of various genes in Arabadopsis thaliana. The transcription factor WRKY70 was found to have a key role in positively regulating SA signaling while suppressing JA responsive genes. "Simultaneous feeding by caterpillars and aphids at a low density induced the expression of the SA transcription factor WRKY70 whereas expression of WRKY70 was lower in plants induced with both caterpillars and a high aphid density." "The expression of JA transcription factor MYC2 was significantly higher in plants simultaneously attacked by aphids at a high density and caterpillars. These results indicate that a lower expression level of WRKY70 leads to significantly higher MYC2 expression through SA-JA crosstalk. Thus, plant-mediated interactions between aphids and caterpillars are density-dependent and involve phytohormonal crosstalk and differential activation of transcription factors." References Kroes, A., J. J. Van Loon, and M. Dicke. "Density-Dependent Interference of Aphids with Caterpillar-Induced Defenses in Arabidopsis: Involvement of Phytohormones and Transcription Factors." National Center for Biotechnology Information. U.S. National Library of Medicine, 22 Oct. 2014. Web. 26 Oct. 2014. . (Main Article) Iwona Morkunas, Van Chung Mai, Beata Gabryś. "Phytohormonal signaling in plant responses to aphid feeding" Acta Physiologiae Plantarum, 2011, p: 2057-2073. 26 Oct. 2014.